As far as I know, neither. It arises with the fertilized egg at conception. The Buddha said that for conception to take place three things must be present – the sexual union of the parents, their fertility and the presence of the gandhabba, ie. the consciousness of the being who is to be reborn (MN I.265). This consciousness absorbs itself in the fertilised egg and begins to animate it so that it grows into a fully formed being. "Consciousness" used here in conventional language, not meant to imply anything permanent; perhaps transference of kammic energies is a better term.

Probably something like: the rebirth consciousness or kammic energies (or whatever you want to call it) from one being "went" to one of the fertilized eggs and another rebirth consciousness went to the other fertilized egg. "It" wasn't there before fertilization when the egg was still one. The rebirth consciousnesses "arrived" after fertilization when there were two, so I suppose could be from two different beings (from a prior life).

Thanks. My understanding is that the split of egg happens after the fertilisation (for identical twins). On the other hand if we talk about a so called (“being “ ie. Gandhabba) aren't we clinging to personality belief (sole theory)?

gandhabba in this context means a being whose kamma enables it to take birth on that occasion, an interpretation supported by a discussion in MN 93.

By introducing a being into the discussion, the Buddha might be suspected of introducing a "what" into his discussion of birth. However, on the level of dependent co-arising, the Buddha did not treat the concept of a being as a "what." His definition of a "being" shows that he recommended that it, too, be regarded as a process:

As he was sitting there, Ven. Rādha said to the Blessed One: "'A being,' lord. 'A being,' it's said. To what extent is one said to be 'a being'?"

"Any desire, passion, delight, or craving for form, Rādha: When one is caught up [satta] there, tied up [visatta] there, one is said to be 'a being [satta].'

"Any desire, passion, delight, or craving for consciousness, Rādha: When one is caught up there, tied up there, one is said to be 'a being.'"

— SN 23.2

Thus the Buddha advocated viewing a "being" simply as a process of attachment to desire, passion, delight, and craving. And it is precisely this attachment to craving that allows for rebirth after death

SarathW wrote:Thanks. My understanding is that the split of egg happens after the fertilisation (for identical twins).

Oh, that's right; it's been a long time since I had a biology class. It is in/after the zygote stage that it splits to two. I guess that could raise more of an issue / problem for soul theories, but since Buddhism doesn't have that, perhaps there is no issue.

Hi SarathW, science reveals new findings every day and from the latest ones like above, it seems like identical twins come from separate gandhabbas. Afterall, even identical twins will grow up and be subjected to different futures according to their own separate kamma. There's no doubt that their old kamma(ie. what they acted, thought, and spoke) in their previous life was very very similar and that's why they share the same kind of initial environment in this current life. However, it can't be exactly the same kamma,ie. one could die abruptly as a result of a car accident in his teenage year while the other might live to a ripe old age. One might grow up to be a wealthy company CEO but married to a nasty woman while the other might grow up to be a modest car mechanic but married to a good wife and have nice children, etc..

How then to account for their behaviour, as observed under microscope? Doesn't seem like merely rupa...

Metta,Retro.

"When we transcend one level of truth, the new level becomes what is true for us. The previous one is now false. What one experiences may not be what is experienced by the world in general, but that may well be truer. (Ven. Nanananda)

“I hope, Anuruddha, that you are all living in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyes.” (MN 31)

SarathW wrote:David: Your comment was very interesting. When I raised this question I thought that identical twins question will be an issue to no soul theory.

Yes, it seems to be more of an issue or problem for those philosophies that advocate a soul. If life / the soul begins at conception and then it later divides into identical twins, what is it? Is it two people sharing the same soul? How does it become 2 souls from one? But no problem with the Buddhist doctrine of anatta.

"And how is it, bhikkhus, that by protecting oneself one protects others? By the pursuit, development, and cultivation of the four establishments of mindfulness. It is in such a way that by protecting oneself one protects others.

"And how is it, bhikkhus, that by protecting others one protects oneself? By patience, harmlessness, goodwill, and sympathy. It is in such a way that by protecting others one protects oneself.- Sedaka Sutta [SN 47.19]

"When we transcend one level of truth, the new level becomes what is true for us. The previous one is now false. What one experiences may not be what is experienced by the world in general, but that may well be truer. (Ven. Nanananda)

“I hope, Anuruddha, that you are all living in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyes.” (MN 31)